Addition of Quetiapine in Obsessive Compulsive Disorder - Westenberg Study

Inclusion Criteria: * All patients meet the DSM IV criteria for obsessive-compulsive disorder * Y-BOCS score \> 16 if obsessions and compulsions * Y-BOCS score \> 10 if only obsessions * Y-BOCS score \> 10 if only compulsions * Male and female, aged between 18-70 years * Female patients of childbearing potential must have a negative pregnancy test and use a reliable method of contraception. * Written informed consent Exclusion Criteria: * Presence of any of the following DSM IV conditions; major depression (with a HDRS\>15, \[17 item\]), bipolar disorder, schizophrenia or any other psychotic condition, tic disorder, substance related disorder during the past 6 months, epilepsy, or any structural CNS disorder or stroke within the last year. * Evidence of clinically significant and unstable cardiovascular, gastro-intestinal, pulmonary, renal, hepatic, endocrine or haematological disorders, glaucoma, myocardial infarction within the past year, or micturition abnormalities * Patients at risk for suicide * Multiple serious drug allergies or known allergy for the trial compounds * Use of antipsychotics during 6 months before the screening visit * Use of any other psychotropic drug during 6 months before the screening visit * Cognitive and behavioural treatment 3 months prior to the screening visit * Any known contra-indication against citalopram or quetiapine